In this unit we will get familiar with the doping objects available in CHARGE solver
to define the doping profile of our device.
The first important point to remember about doping objects is that they are only valid
for semiconductor materials.
Any conductor and insulator that overlaps with the doping objects will not get any doping
value assigned to them.
In addition, always remember that doping objects are additive.
This means that in the areas where two or more doping objects overlap, the net doping
is calculated as the sum of the doping values of all those doping objects.
n and p type doping values are considered to have opposite signs so they subtract from
each other.
The most basic doping object available in a CHARGE simulation is the constant doping
which allows the user to define a region with a constant doping value for the entire region
and is most commonly used to define substrate or background doping.
The geometry settings, which are similar for all doping objects, can be used to define
the areas where the doping will be applied to.
The location and span of the doping object in x,y and z coordinates can be determined
using the corresponding fields.
By default, the doping will be applied to all the simulation domains overlapping with
the defined geometry of the object.
Users have the option to select specific domains or solid objects that they want the doping
to get applied to.
In this case, other areas overlapping with the geometry of the doping object will not
have any doping assigned.
This feature can be used to define non-box shaped doping profiles.
Furthermore, by default, the geometry is defined with respect to the global coordinates origin
(0,0,0).
However, by enabling the relative coordinates options, users have the option to take the
center of the simulation region as origin for the convenience of geometry definition.
Once the geometry of the doping object is defined, the dopant type and concentration
can be specified as well
The diffusion doping object allows the users to create a realistic doping profile using
analytic equations to model doping profile in semiconductors doped using diffusion doping
process.
The geometry settings are same as the constant doping object.
In addition, there are a few parameters in a diffusion doping object that enable the
user to control the doping profile generated by the object.
The dopant type determines whether the doping is ‘p’ or ‘n’ type.
Source face selects the surface through which the dopants enter the doped region.
Junction width determines the length over which the doping concentration gradually reduces
from the peak concentration (near the surface) to the lowest value (inside the semiconductor).
Distribution function selects the analytic function used to describe the diffusion profile.
The two options are Gaussian function and complementary error function.
Concentration describes the peak concentration of dopants introduced at the surface and reference
concentration is a low reference value for concentration that is used as the end point
in the profile.
As illustrated in the diagram, inside the diffusion doping object, the doping concentration
goes from the peak concentration to the ref concentration over a distance equal to the
junction width.
The dark green face is the “mask opening surface,” where the dopants are introduced.
This can be placed on any of the faces by using the “source face” option.
The green box represents the area where the doping concentration is at the peak value
N0.
Outside the box, the dopants start to diffuse into the material towards the outer faces
of the large box (which represents the diffusion doping object).
The green box (the peak concentration area) starts from a certain distance away from all
the faces of the large box.
This distance is the “junction width.”
The doping concentration goes from the “peak concentration” to the “ref concentration”
within this distance.
This diffusion profile is modeled using an analytic function [Gaussian or erfc (complimentary
error function)].
The doping concentration at the faces of the large box (diffusion doping object) is therefore
equal to the “ref concentration” except for the surface where the dopants were introduced
(the top surface here).
Another doping object available is the implant doping object which can model a doping profile
created using an ion implantation process which offers better control over doping profile
than the diffusion process.
The general tab contains parameters that enable the user to control the doping profile generated
by the object.
Under the source section, the properties of the implantation source can be specified which
include dopant type and peak concentration, and elevation and azimuth angles of the source.
A green arrow inside the layout editor will show the implantation orientation when the
object is selected in partitioned volume mode.
The implant distribution profile can also be specified in the same tab.
There are two distribution functions available which include gaussian or pearson4 functions.
The gaussian function takes only two parameters.
Range specifies the location of the peak of the profile or equivalently the average depth
of ion penetration.
Straggle represents the standard deviation of the distribution and is a measure of the
amount of variation in ion penetration depth or equivalently the wideness of the profile.
The pearson4 function which is a more accurate representation of the ion implantation doping
profile takes two additional parameters.
Skewness is a measure of the profile’s tendency to lean forward or backward and a profile
with no skewness has a value of zero.
Kurtosis describes the degree of flatness of the profile.
A perfect gaussian has a kurtosis of 3 and a larger value means the profile is more flat
around its peak.
Lateral scatter describes the standard deviation of the profile in lateral direction which
is used to model the ion penetration beyond the edges of implant mask.
The distribution function for lateral scatter is always gaussian.
A preview of the doping profile with current settings is always shown in the same tab and
the plot range is automatically set by default while the user has the option to choose any
desired ranges for the plot.
Under the geometry tab, the shape and location of the implant mask in the simulation can
be specified and a preview of the location and shape of the mask is always displayed
in the layout editor when the object is selected in partitioned volume mode.
The surface normal determines the axis normal to the surface of the implant mask to specify
its orientation with respect to the structure.
X,Y and Z are used to select the location of the center of the mask.
The list of vertices can be used to define the desired shape for the mask by specifying
their location in the mask’s plane.
Like other doping objects, users have the option to select specific domains or solid
objects that they want the doping to get applied to.
Using the import doping object, a user-defined spatial doping profile can be imported in
the simulation.
The data can be analytic or generated by a process simulation.
The geometry settings are same as the constant doping object with the only difference being
that the span of the object will be determined by the imported data.
Under the Data tab, the file containing the doping profile data and the dopant type can
be specified.
The file must be in Matlab format (.mat extension) and data can be located over a rectangular
or finite element mesh grid.
The unit for the imported doping data should be 1 /m3.
For an example of importing doping profile using this object, please visit the related
links below.